Silymarin is a phytophenol extracted from the seeds of milk thistle (Silibum marianum). Silymarin has been studied in skin UV-protection due to its antioxidant and anti-inflammatory effects. However, its possible phototoxic potential was also shown. Based on these findings the dermatological application of this polyphenol is questionable. 
Our aim was to study the effects of silymarin on cell viability, ROS production and mutagenesis in UVA-irradiated epithelial cells. 
HaCaT immortalized keratinocyte and CHO (Chinese hamster ovary) cell lines were treated with silymarin for 30 min, then exposed to 10 or 20 J/cm2 UVA. Viability, ROS production, cyclobutane pyrimidine dimer (CPD) formation and HPRT gene mutagenesis of the cells were measured by flow cytometry, CPD-specific ELISA and HPRT gene mutation assay, respectively. 
In our experiments the antioxidant effect of the silymarin was demonstrated, but UVA-induced apoptosis was enhanced by silymarin treatment and silymarin showed photosensitizer properties. Silymarin-treated cells showed increased UVA-induced CPDs compared to controls. However, the higher CPD levels did not increase mutagenesis in the HPRT gene mutagenesis assay. 
Our results show that antioxidant and photoprotective effects of chemicals may be dyssynchronous and highlight the complexity of UV-mutagenesis. We also provide evidence for the complex effects of silymarin in modifying UV response and thus the need for careful consideration before using silymarin in dermatological preparations.

Enhanced UVA-induced cyclobutane pyrimidine dimer formation by silymarin without increased mutagenesis in cultured epithelial cells